The proper procedure for running in a cast iron piston and the reasons behind it were discovered by the CL team racing guys with their diesel engines where the fits have to be very precise. Unfortunately the web page no longer exists (or has been moved somewhere) so I'll sum it up.

Fresh cast iron is softer than steel so long as it doesn't get too hot then cooled. This allows the softer piston to lap the harder steel liner without wearing itself. Running slobbering rich keeps the piston as cool as possible and extra oil helps avoid localised hot spots from friction. After the lapping process is finished, shorter and leaner runs with a cool down begins hardening the piston and as it's hardening it also grows slightly. This growth may not be as noticeable in a glow engine where the fit isn't as critical as it is in a competition diesel but why not make use of it?

Cast iron will still do all this even when not subjected to heat cycling as happens in an engine although it takes far longer. High quality (older Rolls Royce for instance) raw engine blocks and rough cast surface plates will be left out in the weather for a year or so to stabilise them before machining.